Abstract

Jacketed cylindrical specimens of dolomite single crystals were deformed dry under a constant confining pressure of 5000 bars at a strain rate of 1 per cent per minute at temperatures from 24°C. to 500°C. Uniaxial compression and extension experiments were carried out with the load oriented (1) parallel to the optic axis Cv, (2) parallel to a horizontal axis a, (3) perpendicular to a cleavage r, and (4) parallel to an edge [f:f].

In orientations (1) and (2), unfavorable for basal translation, crystals loaded at temperatures below 400°C, are brittle and fail on shear fractures inclined at about 30° to the direction of greatest principal pressure. At 400°C, and above, specimens of orientations (1) and (2) flow in compression and extension, respectively. Large permanent deformations are achieved in orientations (3) and (4), favorable for translation gliding, at all temperatures.

Petrographic studies by the techniques developed by Turner in his investigation of calcite crystals (Turner et al., 1954a) reveal that translation gliding on c{0001} along the gliding line parallel to an a axis is the flow mechanism for orientations (3) and (4) below 400°C. This confirms Johnsen's (1902) first discovery and the results of the later work of Turner et al. (1954b) on dolomite rock. The intracrystalline slip mechanism for orientations (1) and (2) is twin gliding on in a negative direction sense (in contrast to the positive sense of twinning in calcite), which, however, occurs only at about 400°C. and above. This confirms the hypothesis of Fairbairn and Hawkes (1941) and the data of Turner et al. (1954b) and Handin and Fairbairn (1955) for dolomite rocks. In orientations (3) and (4) twinning occurs to the exclusion of translation at 500°C., and both mechanisms are operative at 400°C.

The critical resolved shear stress for translation increases with temperature up to 400°C. at least, a most unusual behavior. The critical stress for twinning decreases at least in the range of 400°C. to 500°C., and the two critical stresses are equal at about 400°C.

A few magnesite crystals were deformed parallel to Cv, under similar conditions. Basal translation cannot occur in this orientation, but translation on along the line does occur. No twinning was detected.